1. Field of the Invention
The present invention relates to a laminated glazing comprising at least two rigid substrates joined by an interlayer comprising at least one sheet of a thermoplastic material, in particular, polyvinyl butyral (PVB) . More particularly, the invention relates to a laminated glazing for use as a sliding side window in the body of a motor vehicle, but it is nevertheless not limited to an application of this kind.
2. Background of the Invention
The side windows of motor vehicles are usually fitted by sliding the glass into the body of the vehicle, generally the doors. In order to be fitted in such a way that it can be moved, the window needs to be positioned in the mechanism by which the window is raised and lowered. This mechanism is referred to as the lifting mechanism.
Known methods of positioning of the type mentioned above may, without implying any limitation, be divided into two categories:
A first method of positioning involves a screw assembly. This method requires PA1 the drilling of one or more hole(s) through the window, then screwing two parts PA1 which are integral with the lifting mechanism tight against the two faces of the PA1 window by means of a screw passing through the hole(s). The second method of positioning is that which is described in EP 0 694 669, and involves assembly by gripping. This is done by making two plates bearing on the outer faces of the windows move towards one another, these plates generally forming part of the same U-shaped piece. Specifically, they are moved towards one another by one or more screws which are tightened with a high torque to ensure effective fastening, more particularly when the window is being lowered into the body. This high torque generates a very sizeable force gripping the outer faces of the windows, equal to at least 120 daN. The degree of the torque employed is dictated by the car companies.
There is an increasing demand from the same automobile manufacturing companies, to improve the performance of side windows of an automobile, especially in terms of acoustic insulation and mechanical strength, in particular resistance to break-ins. These factors contribute to the establishment of standards in force, such as regulation No, 43.
It is moreover known that monolithic glazings provide only minor attenuation of acoustic waves, and that the mechanical strength properties of glass windows are greatly enhanced when the glass is toughened. However, in the particular case of side windows, this mechanical strength should not be too high, because, when it becomes desirable to break the windows, for example in the event of an accident, it prevents rapid breaking and, therefore, impedes the possibility of rapid escape by the occupants of the vehicle.
This is why, in order to reach a satisfactory compromise between the abovementioned required properties, laminated glazings, which include glazings of two glass substrates and which have a differentiated level of toughening either from one substrate to another or over the surface of one or both glass substrates, in particular by making the core tensile stress greater in the marginal region of the glazing, have already been proposed, for example in patents EP 0 418 123 and EP 0 560 639.
In order to manufacture such a laminated glazing, it is necessary either to use a thermoplastic sheet of relatively large thickness and/or complex chemical formulation as an interlayer, or to make adequate use of bending/toughening devices designed for the aforementioned differentiated toughening, devices for which fine tuning may prove difficult. In other words, such manufacturing requirements significantly increase the costs of laminated glazings.
It has been found that laminated glazings prepared from two glass substrates having a low degree of toughening over their entire surface, which is to say the core tensile stresses are less than 35 MN/m.sup.2 and which have a thickness .ltoreq. 2.1 mm, and are joined together by a "conventional" interlayer such as a sheet of polyvinyl butyral (PVB), results in a perfect compromise between the properties of mechanical strength and acoustic insulation of the glazing which are desired for side windows which slide in the door of a motor vehicle. Such glazings meet standards such as rule No. 43, particularly from the viewpoint of resistance to breaking. What is more, glazings of this type are easy to manufacture using customary manufacturing techniques, which is advantageous from the industrial viewpoint.
It would, therefore, be beneficial, to substitute the latter type of laminated glazing for the optionally laminated toughened glazing mentioned above as side windows in motor vehicles. The problem which results from this substitution is, quite clearly, that of fitting the laminate glazing in question.
It would, therefore, be very beneficial, both in practical and economic terms, to be able to fit such glazings without major modification either to the frame of the side window or to the mechanism for moving it, particularly with respect to the positioning mechanisms mentioned above, since, among other reasons, it would make it possible to have standardized fittings for all the abovementioned side windows for motor vehicles.
Several types of solution may then be envisaged.
A first type consists in employing the screw assembly mentioned above. For this method a drilling step is required. This step may be carried out before or after the glazing has been laminated by the combined effect of a difference in pressure and heat, for example, in an autoclave. The drilling requirement may lead to a problem of matching up the holes. A second possible problem is that it may be difficult to effectively utilize this method because of the very fact that the thickness of the laminated glazing is small.
A second solution employs the gripping assembly mentioned above, wherein the gripping action is exerted on one of the two glass substrates whose dimensions are larger than the other's, as this configuration is described in EP 0 418 123. However, if the gripping assembly is applied to a substrate made of glass having a low degree of toughening with a thickness .ltoreq. 2.1 mm by means of the fixed tightening torque, this force will be too strong and the substrate will, therefore, fracture.
A third and last solution involves gripping on the sleeve of a rigid plastic piece extended by a U-shaped end part whose branches are bonded to the laminated glazing using an adhesive. However, as well as the fact that this method dictates a particular geometry for the piece which exerts the gripping, care must be taken, on the one hand, that the area of adhesive provided is sufficient and, on the other hand, that no part of the said plastic piece protrudes into the region of the seal which is referred to as the "weather strip", thereby preventing water from entering between the window and the door of the motor vehicle.